Vehicle systems are evolving to meet stricter emissions requirements. Catalytic converters are often employed to help meet these requirements. When operating optimally, a catalytic converter exchanges pollutants including hydrocarbons, carbon monoxide and oxides of nitrogen in the exhausted gas stream to water, carbon dioxide and nitrogen. The catalyst is quite efficient when it is at high temperatures.
One requirement many systems designers are faced with, is how to reduce emissions produced by an internal combustion engine from a cold start condition. When the vehicle is first started after a long inactive period, the catalytic converter is cold and is not effective in removing the excess pollutants from the exhausted gas stream.
To counteract this undesirable result, system designers are turning to approaches that electrically pre-heat the catalytic converter when the vehicle is first started. Since a great deal of energy, typically 125 kJ, is required to pre-heat a typical catalytic converter within the thirty seconds desired, often an auxiliary battery, not the vehicle's main battery, is employed to provide the required energy.
Because batteries only temporarily store energy, after depletion by the catalytic converter pre-heating sequence, the auxiliary battery often needs charging to restore the depleted energy. This charging requirement presents a rather difficult problem to a system supporting it. This is because of the inefficiency associated with charging the auxiliary battery.
FIG. 1 shows a system block diagram of a prior art implementation of a control system for charging auxiliary batteries. In a vehicular application, a charging system 101 typically consists of an alternator driven by a reciprocating engine. This charging system 101, is connected to a primary vehicle battery 103 for the purpose of restoring charge depleted from the primary vehicle battery 103 during normal service by a main load 105. The main load 105 may consist of a starter motor, and other electrical appliances associated with modem vehicles. An auxiliary battery recharge switch 111 is connected between the charging system 101 and an auxiliary battery 107. An auxiliary load enable switch 113 is connected between the auxiliary battery 107 and an auxiliary load 109. The auxiliary load 109 in this instance is an electrical heater for a catalytic converter. The auxiliary battery recharge switch 111 and the auxiliary load enable switch 113 are operated by a control device 115. Generally, the switches 111 and 113 are activated exclusively when the vehicle is operating.
Operation of the system is as follows. When a vehicle having this system is initially turned on, the control device 115 disconnects the auxiliary battery 107 from the charging system 101 by opening the auxiliary battery recharge switch 111 via control line 117. Also, the control device 115 closes the auxiliary load enable switch 113 via control line 119. Closing the auxiliary load enable switch 113 connects the auxiliary battery 107 to the auxiliary load 109. When this occurs the auxiliary load 109, in this case an electrical heater for a catalytic converter, is activated and thereby partially discharges the auxiliary battery 107 via a discharge current 123 while the heater dissipates heat. When a pre-heat period is finished, the control device 115 will open the auxiliary load enable switch 113, thereby disconnecting the auxiliary battery 107 from the auxiliary load 109 and interrupting the discharge current 123.
At this juncture the vehicle has started operating and its reciprocating engine is driving the charging system 101. Also, the main battery 103 is being charged by the charging system 101. The control device 115 then closes the auxiliary battery recharge switch 111 via control line 117 thereby creating a charge current 121 for recharging the auxiliary battery 107. When the auxiliary battery recharge switch 111 is initially closed, the auxiliary battery 107 will have a lower terminal voltage across it than the charging system 101. Because both the auxiliary battery 107 and the charging system 101 have relatively low terminal resistance, the voltage across the auxiliary battery recharge switch 111 is dependent on these terminal voltages. If the auxiliary battery 107 is substantially depleted, then the voltage across the auxiliary battery recharge switch 111 can be substantial. Because the auxiliary battery recharge switch 111 has some finite resistance, when the charge current 121 is active, the auxiliary battery recharge switch 111 will have a relatively high power dissipation. For instance, if the auxiliary battery recharge switch 111 has a resistance of 30 milliohms, and the current of the charge current 121 is typically about 50 amps the power dissipated by the auxiliary battery recharge switch 111 will be about 75 watts. Providing a switch for dissipating 75 watts in a vehicular environment is not only costly but because the operating temperature environment often exceeds 85 degrees Celsius it is difficult to design a reliable, long lived switch device to function as the auxiliary battery recharge switch 111. Further, since the power dissipation of the auxiliary battery recharge switch 111 will vary dependent on the voltage across the auxiliary battery recharge switch 111, the charge current 121 must be limited by the maximum power dissipation of the auxiliary battery recharge switch 111. This means that, pursuant to the protection of the auxiliary battery recharge switch 111, only a fixed charge current 121 will be used to charge the auxiliary battery 107. Because of this fixed charge current 121, the auxiliary battery 107 will take a relatively long time to charge.
A further defect in prior art systems is that when the charging system 101 is heavily loaded there is no facility to impede the addition of further loading associated with recharging the auxiliary battery 107.
What is needed is an improved control system for charging an auxiliary battery in a multiple battery system, that is not only more efficient but more reliable than prior art schemes.